Conventional wireless transmitters generally include a mix of digital and analog components. Typically, a digital baseband modulates the signal into a passband for analog amplification and transmission. To achieve a desired level of performance, it is often necessary to adjust the power at which the signal is transmitted. Although increasing signal power can improve reception, maximizing power is not always desirable. For example, Federal Communications Commission (FCC) regulations cap permissible transmit powers. Further, in Bluetooth applications, error vector magnitude (EVM) and adjacent channel power (ACP) metrics can be degraded when transmit power is too great. One technique for controlling the signal transmission power is to provide a gain table that correlates transmission power with the parameters necessary to control the amplifier and related analog components. Thus, once a desired transmit power is determined, the baseband will lookup the appropriate parameters from the gain table and apply them to the transmitter.
As one of skill in the art will recognize, the behavior of analog components varies depending upon environmental conditions, such as temperature. If these variations are too great, the signal will not be transmitted at the desired power and performance of the system will suffer.
One method for compensating for variations in analog performance is to use an external power detector to monitor the transmitted signal power. By comparing the difference between the measured transmit power from the external power detector and the desired power, the baseband can adjust its access to the gain table to drive transmitted power closer to the desired power. Such methods are generally known as closed loop power control.
Despite the advantages of providing direct information about the transmitted power, the necessity of supplying an external power detector adds to the cost and complexity of the hardware. Indeed, the costs associated with wireless integrated circuits have decreased to the point at which the external power detector represents a significant fraction of the expense. Further, the addition of the parameters associated with the difference between measured power and desired power increases the size of the gain table significantly, requiring greater onboard memory and slowing access to the table.
Accordingly, what has been needed is a system and method for controlling transmit power in communication systems that avoids the need for an external power detector. Similarly, there is a need for a system and method of controlling transmit power that compensates for environmental variations, such as temperature, to produce a transmit signal at a desired power level. This disclosure is directed to these and other needs.